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Programming Note 13 Page 1
BASIC FORTRAN PLOTTING SUBROUTINES
The subroutines described below manipulate a "pen" which can
be moved about, raised or lowered, to create a line drawing on
paper. Pen movements are made in a straight line from one point
to another, relative to an origin which can be defined. Curves
are made up by drawing a number of small straight line segments.
Subpictures can be created by writing a subroutine which
redefines the origin to a given point, draws the subpicture, and
restores the original origin. There are subroutines to aid graph
drawing, which will scale an array of data to fit a given graph
size, draw axes, and draw the points. Other subroutines will
draw text messages and magnify a part of the plot and set and
return some parameters like the current pen position.
These functions are primitive operations from which the
programmer can build his own more complicated drawings. Other
FORTRAN callable plotting routines are ATOPLT, which is a simple
method of drawing graphs described in PD25, and others which may
be found in the Application Software Catalogue.
1.1 Plotting Hardware At La Trobe
The plotter is a Gould electrostatic printer/plotter. It
does not have a physical pen like an incremental plotter but can
operate at high speed producing medium quality drawings on
electrostatic paper. The plotting software makes the Gould look
like an incremental plotter but allows the user to make use of
some special features on the Gould, such as the ability to erase
lines already drawn and to draw lines of different densities.
The plotter paper is 250mm wide and the maximum plot length
is about 3m. The plotting software will allow the user to draw
plots up to 3m wide by breaking the plot into a number of strips
250mm wide, which can be joined by the user to give the full
sized plot. Users who wish to use this feature should consult
the Computer Centre staff first as it is a marvellous way to
waste expensive paper.
The computer centre is writing routines to enable these
basic FORTRAN subroutines to be used with the Tektronix 4012
terminal. This is the subject of a separate note.
Programming Note 13 Page 2
1.2 Plot Spooling
Subroutines do not draw directly onto the plotter, but
create disk files which must be queued to the plotter. The files
produced by the plotter subroutines have a random name and
extension .PLT. At logout the KJOB command queues all .PLT files
to the plotter, but the user may queue the files beforehand using
the PLOT command (see User's Handbook), e.g.
All plotting is done in millimeters. This means that if you
request, by a call to some subroutine, a line or pen movement of
say W units, the actual distance on the paper covered by the line
or pen movement will be W mm. This holds true unless the user
specifies a scaling factor (or magnification factor) by a call to
the subroutine FACTOR. If a scaling factor of say Z is specified
for the X direction on the paper then a call for a move of W
units in the X direction will result in a move of W*Z mm in the X
direction on the paper. These scaling factors, once set, hold
for all lines drawn by any plotting subroutine. The units
specified by the user in a call are refered to as USER UNITS, as
opposed to the scaled physical units of movement.
2.0 PLOTTING SUBROUTINE CALLS.
FORTRAN naming conventions are used throughout this document
to indicate whether a variable is REAL or INTEGER. Optional
arguments are indicated by inclusion in '' (which are not part
of the argument string) and may be included or omitted at the
wish of the user.
The subroutine calls are similar to a set of calls known as
CALCOMP subroutine calls and widely used as a de facto industry
standard. The differences between the La Trobe calls and CALCOMP
calls are detailed in Appendix D.
2.1 Initializing A Plot.
This routine must be called BEFORE any other plotting
routine. It creates the output file and initializes internal
variables. Failure to call this routine will result in the fatal
error message 'PLOT FILE NOT OPEN'.
The paper sizes specified in this call (PX and PY) are the
expected true size of the plot, that is the physical size of the
paper used by the plot. If this size is exceeded by a plot then
the user program will by stopped and the message
Programming Note 13 Page 3
'X COORDINATE OUT OF BOUNDS'
'Y COORDINATE OUT OF BOUNDS'
will be printed.
This subroutine moves the pen to (0.0,0.0) the paper and
sets the scaling factors to 1.0.
CALL PLOTS (PX,PY,IBAK)
PX X paper size in millimeters
PY Y paper size in millimeters
IBAK background code
IBAK = -1 is black paper with white lines
= 0 is white paper with black lines
2.2 Moving The Pen
This subroutine allows the user to move the pen from its
current position to that specified by the arguments. See 2.5 for
details of scaling and 2.3 for details of origin moving, both of
which effect the result of a call to PLOT. The routine also
allows for the pen to be raised, lowered or left as it is before
the required move is performed.
CALL PLOT (X,Y,IPEN)
(X,Y) the coordinate (in user units) of the point to
which the pen is to move
IPEN the code to indicate the state to which the pen
must be set before the movement
IPEN = 3 pen up
IPEN = 2 pen down
IPEN = 1 no change
IPEN = 0 erase line
2.3 Moving The Origin
All coordinates given are relative to a user defined origin.
The origin is defined to be (0.0,0.0), the left hand bottom
corner of the paper. It can be moved by calling PLOT with a
negative value for IPEN. The pen is placed on the new origin.
CALL PLOT (X,Y,IPEN)
(X,Y) the new origin relative to the old origin
IPEN the code to indicate the state of the pen before
the move to (X,Y)
IPEN = -3 pen up during move
IPEN = -2 pen down during move
IPEN = -1 no change to pen before move
Programming Note 13 Page 4
2.4 Closing A Plot.
Before stopping, a user program must close the plot file.
If this is not done the output may be lost. This is achieved by
a call to PLOT.
CALL PLOT (X,Y,999)
999 indicates that any open plot file should be
2.5 Altering The Scaling Factors.
Sets the scaling (magnification) factors for the plot. The
factors are multiplicative. For instance if you wish a call for
a movement of M units to correspond to 0.5M millimetres of paper
a scaling factor of 0.5 should be employed:
i.e. (user units) X (scaling factor) = (millimetre moved).
CALL FACTOR (XFACT[,YFACT])
XFACT X-scaling factor, and Y scaling factor unless the
YFACT argument is given
YFACT Y-scaling factor, if given
2.6 Plotting Text
The subroutine SYMBOL allows a program to plot a string of
ASCII characters. The string may be located at any point in the
plot and be drawn at any angle to the X direction. The height of
the characters can be specified. The width of the character plus
the inter-character space is equal to the height. The pen is
positioned at the lower left hand corner of the next character
position on return from SYMBOL (see appendix C for a table of the
characters as they are drawn).
CALL SYMBOL (X,Y,HEIGHT,TEXT,ANGLE,NCHAR)
(X,Y) position of lower left hand corner of text
HEIGHT height of characters in user units
TEXT text to be plotted, either a literal or an array
ANGLE the angle of the text to the X direction in
degrees, anticlockwise rotation.
NCHAR 0 a single character, in R1 format, is plotted
+N N is the number of characters, in A5 format,
Programming Note 13 Page 5
2.7 Plotting Special Centering Symbols
There are a number of symbols available that are not part of
the standard ASCII character set. These symbols are plotted by
the subroutine SYMBOL and are centred on the coordinates given.
The symbol to be plotted is represented by an integer value in
the calling sequence. Appendix C lists the symbols available and
their corresponding integers. The pen is left at the centre of
the symbol after it is drawn.
CALL SYMBOL (X,Y,HEIGHT,INTEG,ANGLE,ICODE)
(X,Y) centre of the symbol
HEIGHT height of the symbol
INTEG integer corresponding to the desired symbol
ANGLE angle of symbol to X direction in degrees
ICODE -3 pen up for move to (X,Y)
-2 pen down for move to (X,Y)
2.8 Plotting An Axis
This subroutine draws an axis according to the
specifications in the call, with ticks every 10 user units,
titles and value labels. The values of the arguments XMIN and DX
can best be obtained by calling the routine SCALE (see section
CALL AXIS (X,Y,ASC,NASC,S,THETA,XMIN,DX)
X X coordinate of start of axis
Y Y coordinate of start of axis
ASC axis label, either literal or array
NASC number of characters in ASC. If NASC is positive
ticks are on clockwise side of axis and if
negative they are on anticlockwise side
S length of axis in user units
THETA angle of axis to X direction, in degrees
XMIN value of label to be drawn at the first tick on
DX the amount by which the label drawn at each tick
is incremented per tick
2.9 Drawing Lines Through Points
Draws a continuous set of straight line segments between
CALL LINE (X,Y,N,K)
X array of X data
Y array of Y data
N number of points to be drawn
K the increment to be used to select values from
the X and Y array. For instance if K=2 only
every second point in the X and Y arrays will be
Programming Note 13 Page 6
2.10 Plotting Numbers
This routine plots a number in FORTRAN F type format except
that no limit is given for the total number of digits in the
CALL NUMBER (X,Y,SIZE,FNUM,THETA,NDIG)
X X coordinate of start of number (lower lefthand
Y Y coordinate of start of number (lower lefthand
SIZE size of each digit in user units
FNUM number to be plotted
THETA angle of number to X direction in degrees
NDIG number of digits to be plotted to right of
decimal point (if negative, the integer part only
2.11 Scaling Points For Plotting
The subroutine SCALE is passed an array of data (X) and the
length of the axis on which the data is to be plotted. SCALE
selects a suitable minimum value (XMIN) and increment per tick
(DX) from the array, so that the data may be plotted on the axis.
The values of XMIN and DX are returned and should be used in a
call to AXIS so that the axis drawn fits the data in array X.
These values are chosen in such a way as to eliminate rounding
error from the plotted tick values. The contents of the array
are scaled to allow them to be plotted against the axis with a
call to LINE. SCALE normally changes the contents of the array
CALL SCALE (X,N,S,XMIN,DX)
X the data array
N number of elements in X
S axis length
XMIN scaled data minimum for use by AXIS
DX scaled data increment for use by AXIS
2.12 Locating The Pen
Returns the current setting of the variables in the calling
CALL WHERE (X,Y,XFACT[,YFACT])
(X,Y) pen position in user units relative to the
XFACT current X scaling factor
YFACT current Y scaling factor
Programming Note 13 Page 7
2.13 To Change Line Density
The plotter is capable of plotting lines of various density.
Every nth point in the line is plotted to achieve a line density
of 1/n. This setting, which is applied to characters and erases
also, holds until the next call to LINEWT.
CALL LINEWT (I)
I density indicator
0 or 1 standard
-n 1/n line density
Programming Note 13 Page 8
3.1 Example Of Use Of PLOTS, PLOT, FACTOR And SYMBOL.
See APPENDIX A for output.
C INITIALIZE THE PLOT
C REDEFINE THE ORIGIN
C SET THE SCALING FACTORS
C FOR X 1 UNIT = 50. MM
C FOR Y 1 UNIT = 50. MM
C NOW DRAW A BOX WITH SYMBOLS AT THE
C NOW TO DRAW THE SYMBOLS 2.0 MM TO THE RIGHT OF
C AND ABOVE EACH CORNER.
C CLOSE THE PLOT FILE
Programming Note 13 Page 9
3.2 Use Of SCALE, AXIS And LINE.
See APPENDIX B for output.
DATA X /1.,2.,3.,4.,5.,6.,7.,8.,9.,10.,11.,12.,
DATA Y /.01,.02,.03,.04,.05,.06,.07,.08,.09,.1,
C INITIALIZE THE PLOTTING SOFTWARE
C SCALE THE X DATA
C SCALE THE Y DATA
C DRAW THE X AXIS
CALL AXIS(30.0,30.0,'X AXIS',-6,150.,0.0,XMIN,DX)
C DRAW THE Y AXIS
CALL AXIS(30.0,30.0,'Y AXIS',6,150.,90.0,YMIN,DY)
C SET THE ORIGIN TO WHERE THE AXIS CROSS
C PLOT THE DATA
C CLOSE THE PLOT FILE
Programming Note 13 Page 10
Plot Produced by Example 1.
Programming Note 13 Page 11
Plot Produced by Example 2.
Programming Note 13 Page 12
Programming Note 13 Page 13
Converting CALCOMP Calls to La Trobe.
This section explains how to modify a program using CALCOMP
Plotting Subroutines to work at La Trobe. Full details of the
CALCOMP software is available in 'PROGRAMMING CALCOMP PEN
PLOTTERS' which may be borrowed from the reception desk of the
Calls to this routine are plotter dependent and
must be replaced. Simply replace the CALCOMP call with
a suitable La Trobe call.
No changes are needed to calls to PLOT.
No changes are needed to calls to FACTOR.
No changes are needed to calls to WHERE.
No changes are needed to calls to AXIS.
This subroutine has no exact equivalent on the La
Trobe system. Similar effects can sometimes be obtained
by varying the line weight (CALL LINEWT) on the La Trobe
plotter. Therefore a user may remove calls to NEWPEN or
replace them with suitable calls to LINEWT.
The only differences between the CALCOMP and La
Trobe SYMBOL subroutines occur in the call to draw
centred symbols. The CALCOMP call uses -1 for pen up in
move to (X,Y) and -2 for pen down. Change -1 in the
CALCOMP call to -3 and the two routines will perform the
The La Trobe SYMBOL routine does not have the
continuation feature. That is, if the call has 999.0
for the X and Y arguments then the text will be
continued after the last text drawn. The following
example will show how to simulate this effect at La
Programming Note 13 Page 14
CALL SYMBOL (10.0,20.0,3.0,'EXAMP',0.0,5)
CALL SYMBOL (999.0,999.0,3.0,'LE',0.0,2)
CALL SYMBOL (10.0,20.0, 3.0,'EXAMP',0.0,5)
CALL WHERE (SX,SY,FACT)
CALL SYMBOL (SX,SY,3.0,'LE',0.0,2)
No changes are required to the calls to NUMBER, but
the result may be slightly different. In the CALCOMP
call the number-of-digits argument equal to -n means
truncate n digits from the right of the number. At La
Trobe it means the same as -1, that is draw the integer
part of the number only.
In the CALCOMP call the order and number of the
arguments is different. The second and third arguments
are reversed and the increment is not available at
For the CALCOMP call the values of XMIN and DX are
returned in the data array, so that the data array must
be two elements longer than the number of data elements
in it. It is necessary to modify CALCOMP calls to
include XMIN and DX in the arguement list and to use
these values later, e.g. in calls to AXIS.
Other more serious difficulties may arise as a
result of the following
1. The La Trobe call does not allow the user to specify
an increment for the arrays (so that only every nth
element is used). This may result in the need to
copy the data to be scaled into a seperate array
before calling SCALE.
2. The sign of INC in the CALCOMP call indicates
whether XMIN is to be a maximum or a minimum and DX
negative or positive respectively. This feature
does not exist in the La Trobe system.
Programming Note 13 Page 15
The CALCOMP call:
CALL SCALE (X,AXLEN,N,INC)
X data array, where X is 2 elements longer than
the length of the data
AXLEN axis length
N size of X
INC increment to be used when choosing elements of X
that is if INC = 2 only select every second
element of X
CALL SCALE (X,200.0,100,2)
CALL AXIS (50.0,50.0,'CALCOMP',7,200.0,
C COPY REQUIRED DATA INTO AN ARRAY
DO 10 I = 1,100
10 X1(I) = X(2I-1)
CALL SCALE (X1,100,200.0,XMIN,DX)
CALL AXIS (50.0,50.0,'LA TROBE',8,200.0,XMIN,DX)
The CALCOMP call to LINE allows the drawing of
symbols at the points being plotted, optionaly joined by
straight line segments. These features are not
available with the La Trobe LINE routine and must be
programmed (see examples below).
Programming Note 13 Page 16
The CALCOMP call is:
CALL LINE (X,Y,N,INC,LTYP,INT)
X & Y data arrays
N number of points to be plotted
INC the increment in the array between points to be
LTYP magnitude of LTYP determines the frequency of
plotted symbols i.e. if LTYP = 4 the symbol
specified by INT is drawn at every 4th data
point; if LTYP = 0 points are connected by
straight line segments and no symbols are drawn;
if LTYP > 0 straight lines connect every data
point; if LTYP < 0 no connecting lines are
INT integer specifying the centering symbol to be
drawn at the points
To replace a call to the CALCOMP LINE routine the
following will be necessary.
(a) if LTYP = 0 include the La Trobe call to LINE
CALL LINE (X,Y,N,INC)
(b) if LTYP > 0 replace the CALCOMP call with the
J = 0
CALL PLOT (X(1),Y(1),3)
DO 10 I=1,N,INC
J = J + 1
IF(MOD(J,LTYP).NE.1) CALL PLOT(X(I),Y(I),2)
(c) if LTYP < 0 replace the CALCOMP call with the
J = 0
DO 10 I=1,N,INC
J = J + 1
IF(MOD(J,LTYP).NE.1) GO TO 10
CALL SYMBOL (X(I),Y(I),SIZE,INT,0.0,-3)